Commit e85265e8 authored by Josh Bleecher Snyder's avatar Josh Bleecher Snyder

cmd/compile: optimize bool to int conversion

This CL teaches SSA to recognize code of the form

// b is a boolean value, i is an int of some flavor
if b {
	i = 1
} else {
	i = 0
}

and use b's underlying 0/1 representation for i
instead of generating jumps.

Unfortunately, it does not work on the obvious code:

func bool2int(b bool) int {
	if b {
		return 1
	}
	return 0
}

This is left for future work.
Note that the existing phiopt optimizations also don't work for:

func neg(b bool) bool {
	if b {
		return false
	}
	return true
}

In the meantime, runtime authors and the like can use:

func bool2int(b bool) int {
	var i int
	if b {
		i = 1
	} else {
		i = 0
	}
	return i
}

This compiles to:

"".bool2int t=1 size=16 args=0x10 locals=0x0
	0x0000 00000 (x.go:25)	TEXT	"".bool2int(SB), $0-16
	0x0000 00000 (x.go:25)	FUNCDATA	$0, gclocals·23e8278e2b69a3a75fa59b23c49ed6ad(SB)
	0x0000 00000 (x.go:25)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0000 00000 (x.go:32)	MOVBLZX	"".b+8(FP), AX
	0x0005 00005 (x.go:32)	MOVBQZX	AL, AX
	0x0008 00008 (x.go:32)	MOVQ	AX, "".~r1+16(FP)
	0x000d 00013 (x.go:32)	RET

The extraneous MOVBQZX is #15300.

This optimization also helps range and slice.
The compiler must protect against pointers pointing
to the end of a slice/string. It does this by increasing
a pointer by either 0 or 1 * elemsize, based on a condition.
This CL optimizes away a jump in that code.

This CL triggers 382 times while compiling the standard library.

Updating code to utilize this optimization is left for future CLs.

Updates #6011

Change-Id: Ia7c1185f8aa223c543f91a3cd6d4a2a09c691c70
Reviewed-on: https://go-review.googlesource.com/22711
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: default avatarKeith Randall <khr@golang.org>
parent c7b9bd74
......@@ -57,7 +57,16 @@ func phiopt(f *Func) {
}
for _, v := range b.Values {
if v.Op != OpPhi || !v.Type.IsBoolean() {
if v.Op != OpPhi {
continue
}
// Look for conversions from bool to 0/1.
if v.Type.IsInteger() {
phioptint(v, b0, reverse)
}
if !v.Type.IsBoolean() {
continue
}
......@@ -110,5 +119,55 @@ func phiopt(f *Func) {
}
}
}
}
func phioptint(v *Value, b0 *Block, reverse int) {
a0 := v.Args[0]
a1 := v.Args[1]
if a0.Op != a1.Op {
return
}
switch a0.Op {
case OpConst8, OpConst16, OpConst32, OpConst64:
default:
return
}
negate := false
switch {
case a0.AuxInt == 0 && a1.AuxInt == 1:
negate = true
case a0.AuxInt == 1 && a1.AuxInt == 0:
default:
return
}
if reverse == 1 {
negate = !negate
}
switch v.Type.Size() {
case 1:
v.reset(OpCopy)
case 2:
v.reset(OpZeroExt8to16)
case 4:
v.reset(OpZeroExt8to32)
case 8:
v.reset(OpZeroExt8to64)
default:
v.Fatalf("bad int size %d", v.Type.Size())
}
a := b0.Control
if negate {
a = v.Block.NewValue1(v.Line, OpNot, a.Type, a)
}
v.AddArg(a)
f := b0.Func
if f.pass.debug > 0 {
f.Config.Warnl(v.Block.Line, "converted OpPhi bool -> int%d", v.Type.Size()*8)
}
}
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